The recent discovery of superconducting metal oxides such as YBa.sub.2 Cu.sub.3 O.sub.7-y (with y&lt;0.5) having superconducting transition temperatures at least 10 to 20K above the temperature of liquid nitrogen (77K) has created a great deal of excitement. As known in the art, the superconducting transition temperature (T.sub.sc) of a material is that temperature below which the material has essentially zero electrical resistance. Heretofore known superconducting materials have much lower T.sub.sc 's. These superconducting metal oxides have vast potential for use in diverse applications in a large number of electrical and electronic devices which can operate at these higher temperatures.
Generally, in making such superconducting materials in the Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-8 system, a metal oxide precursor material is first formed and then subjected to an anneal in oxygen to make it superconducting. The typical annealing schedule consists of a relatively slow heating to a peak temperature in the range of 850.degree. to 950.degree. C., annealing in oxygen at this temperature for a time period ranging from a few minutes to several days, followed by a slow cooling at a rate of 1 to 3 degrees per minute in O.sub.2. It is believed that oxygen atoms diffuse into the lattice structure when the material cools down through the temperature range of about 600.degree.-450.degree. C. These materials, having transition temperatures around 90K, are stable at room temperature in air and retain their superconducting properties (transition temperatures around 90K) even through repeated thermal cycling between 4K and 300K. This is believed due to the fact that the oxygen is "locked" in the crystal lattice.
A great deal of effort has been expended in the last 24 months to make superconducting materials having even higher transition temperatures than those described above. Currently, the highest superconducting transition temperature for well established CuO-based materials is 125K in the TlBaCaCuO system, although evidence for superconductivity/zero resistance at temperatures above this temperature in CuO-based materials has been reported in numerous publications and communications since February 1987, including the following papers: J. T. Chen, L. E. Wenger, C. J. McEwan, and E. M. Logothetis, "Observation of the reverse ac Josephson effect in YBaCuO at 240K", Physical Review Letters, Vol. 58, No. 19, pp. 1972-1975, 11 May 1987; C. Y. Huang, L. J. Dries, P. H. Hor, R. L. Meng, C. W. Chu, and R. B. Frankel, "Observation of possible superconductivity at 230K", Nature, Vol. 328, pp. 403-404, 30 July 1987; H. Ihara, N. Terada, M. Jo, M. Hirabayashi, M. Tokumoto, Y. Kimura, T. Matsubara, and R. Sugise, "Possibility of superconductivity at 65.degree. C. in SrBaYCuO system", Japanese Journal of Applied Physics, Vol. 26, pp. L1413-L415, 1987; J. Narayan, V. N. Shukla, S. J. Lukasiewicz, N. Biunno, R. Singh, A. F. Schreiner, and S. J. Pennycook, "Microstructure and properties of Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.9-.delta. superconductors with transitions at 90 and 290K", Applied Physics Letters, Vol. 51, No. 12, pp. 940-942, 21 September 1987; and H. D. Jostarndt, M. Galffy, A. Freimuth, and D. Wohlleben, "Unstable high temperature superconductivity and martensitic effects in YBaCuO", Solid State Communications, Vol. 69, No. 9, pp. 911-913, March 1989. These papers teach that although previous materials showing higher transition temperatures (greater than 90K) were prepared by conventional ceramic techniques, the materials all suffer from the lack of stability and reproducibility. That is, the zero-resistance state (the ability to pass current without any voltage drop or power loss) in these CuO-based superconducting materials at these higher transition temperatures is a temporary (usually one time) phenomenon which may exist for only a few thermal cyclings of the sample through its higher transition temperature, even though the lower 90K superconducting transition remains.
In addition, several papers report that exposure of certain CuO-based superconductors to a gas during electrical measurements at cryogenic temperatures (77K to 150K) can enhance their 90K superconducting transition temperature. These papers include: D. N. Matthews, A. Bailey, R. A. Vaile, G. J. Russell, and K. N. R. Taylor, "Increased transition temperature in Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.y superconducting ceramics by exposure to nitrogen", Nature, Vol. 328, pp. 786-787, 27 August 1987;K. N. R. Taylor, A. Bailey, D. N. Matthews, and G. J. Russell, "Enhancement of T.sub.c in Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-.delta. superconductors by gas absorption", Physica C, Vol. 153-155, pp. 349-350, 1988; D. N. Matthews, A. Bailey, T. Puzzer, G. J. Russell, J. Cochrane, R. A. Vaile, H. B. Sun, and K. N. R. Taylor, "Effects of helium absorption on the superconducting mechanism of Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.y ", Solid State Communications, Vol. 65, No. 5, pp. 347-350, February 1988; and X. Granados, M. Carrera, J. Fontcuberta, M. Vallet, and J. M. Gonzalez-Calbet, "On the effects of helium absorption on the superconducting onset of Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-y ", Solid State Communications, Vol. 69, No. 11, pp. 1073-1077, March 1989. The cited reference in Physica C teaches that the superconducting transition temperature for a single-phase Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 superconductor material can be enhanced to a maximum of 105K when exposed to O.sub.2 gas for a few hours at 77K, although the zero resistance still occurs at 90K.
Extensive research has been carried out in an attempt to provide a "stable", higher transition temperature superconducting phase material because of its potential for applications in the electronic/electrical industries. It is an object of the present invention to provide a method for making a material that exhibits zero resistance and for maintaining its zero resistance property. It is a further object of this invention to provide a method for maintaining the zero resistance property of still other copper oxide based materials.